Recent years, in a thinning display trend, has witnessed wide use of liquid crystal displays (LCDs for short) in various electronic products, such as mobile phones, laptops, and color televisions.
Because liquid crystals per se do not emit light, nor does an LCD panel itself have a light-emitting property, a lighting source, such as a backlight module, is necessary for an LCD. An image forms on the LCD when light from the backlight module passes through liquid crystal molecules, which will twist upon energization of electrodes arranged in the LCD panel. A backlight is an optical component providing a light source at a back face of an LCD product. Therefore, the quality of the backlight will determine important parameters of the LCD product, such as brightness of the display screen, homogeneity of exit light, and color gradation, thereby largely determining lighting effects of the LCD product.
As backlights of LCDs, light-emitting diodes (LEDs) have shown a trend in replacing cold cathode fluorescent lamps (CCFLs). LEDs have the advantages of wide color gamut, superior color reducibility, strong controllability, long service life, free of mercury vapor and other harmful gases, etc. Since an LED is a low-voltage, non-linear semiconductor device, the positive voltage thereof would vary with electric currents and temperatures. This renders a drive circuit necessary to ensure stable and reliable work of the LED. Therefore, study of LED drive circuits used in backlights is a major task to which those skilled dedicated.
FIG. 1 schematically shows a backlight drive circuit in the prior art. As illustrated in FIG. 1, a constant voltage V1 of a constant-current LED driver IC, and a resistor R arranged at a negative terminal of an LED string together determine the size of a current amplitude flowing through the LED string, i.e., I=V1/R. An operational amplifier arranged inside the constant-current LED driver IC can feedback the voltage applied to the resistor R, and then determine the value of a current flowing between the source and the drain of a MOS transistor Q2 located at the negative terminal of the LED string, i.e., the current flowing through the LED string, via adjustment of activation or deactivation of the MOS transistor Q2.
However, the constant-current LED driver IC has limited capacity in adjusting currents due to a manufacturing procedure of a semiconductor and price factors. As a result, a predetermined current value will not be achieved through adjustment by the constant-current LED driver IC beyond a specific range. In addition, when the above circuit is used, the value of the current flowing through the LED would be subject to relatively large errors with respect to the predetermined current value.
Therefore, it is a major issue in the art to solve the above problem, so as to adjust the current flowing through the LED to a predetermined value with reduced errors.